1. Field of the Invention
The present invention relates to an image forming apparatus with a plurality of color. In particular, the invention relates to an electrophotographic copying machine which can selectively form a different color image from that of the other portions by designating a required area of an original document.
2. Discussion of Background
As already known, various electronic copying machines have been developed by which it is possible to designate a required portion of an original document placed on the original document table using a designating means such as a spot light source and to develop that portion using a different color from other portions.
In the above image forming apparatus, as shown in FIG. 1A, when a first exposure scan is carried out, for instance, an image is developed which is the portion outside the area of an original document G which has been designated (that portion shown by diagonal shading). While the electrostatic latent image of the designated area is erased from the photosensitive drum using an erasure means of charging which is a so-called erasure array so that the portion erased from the photosensitive drum will not be developed. After this developed image has been transferred onto a sheet, the sheet is guided to a sheet transport device which is called a multicopying unit via a fixing unit.
On the other hand, when a second exposure scan is carried out, only the image of the designated portion (that portion shown by diagonal shading) is developed. While the image of the portion other than the designated portion is erased by the erasure array so that it is not developed. Then, this developed image is transferred onto the sheet on to which the image of other than the designated area has been transferred, which is supplied from the multicopying unit. By carrying out such operations as these, the required portion can be copied in the required color.
However, when changing the developing color of an original document image by partial designation, as shown in FIG. 1B, there are cases when a change of the developing color is designated for the whole image in the direction (the x direction) which is orthogonal to the exposure-scanning direction (the y direction) of original document G. In the past, even when this type of designation was carried out, the image had to be copied by carrying out two exposure scans and developing the designated portion and portion which was not designated by separate processes as described above. For this reason, copying took long time. Also, since the sheet passed through the fixing unit in the first image formation process, curling of the sheet occurred and, in the second image formation process, it was difficult accurately to place it in contact with the reference position on the photosensitive drum when the sheet was supplied from the multicopying unit. Thus there was the drawback of position slippage occurring between the image formed in the second image formation process and the image formed in the first image formation process.